A pressure sensitive adhesive (PSA), generally, is a material which adheres to a surface with slight pressure and releases from the surface with negligible transfer of the material to the surface. Silicone pressure sensitive adhesives that are known in the art are typically solvent based adhesives; the solvents are employed primarily to reduce the silicone pressure sensitive adhesive's viscosity to a viscosity which is easily coated onto the substrate of choice, and the solvents are removed after coating.
Hot-melt pressure sensitive adhesives are those adhesives, which upon heating, melt to viscosities suitable for coating, but when cooled are generally in a flowless state. The advantages of hot-melt PSA's relative to solvent-based PSA's are known and include safety, environmental and application advantages. In addition, hot-melt PSA's have the advantage of not containing solvents which sometimes interfere with the addition of other ingredients to the PSA. Silicone pressure sensitive adhesives have been found to be preferred over other types of PSA's in many applications, especially in the medical area. Pressure sensitive adhesives have long been used to bind bandages, sensory monitors, and the like to a person's skin. In addition, silicone pressure sensitive adhesives have found use in transdermal drug delivery applications which involve the adherence of a drug-containing patch to a patient's skin.
U.S. Pat. No. 4,865,920 discloses a hot-melt PSA comprised of (i) a silicone resin, (ii) a silicone fluid and (iii) an ester having the formula R'--C(O)OR" wherein R' is a monovalent hydrocarbon radical having from 2 to 32 carbon atoms and R" is a monovalent hydrocarbon radical having from 1 to 14 carbon atoms.
Other additives, besides the esters, have recently been developed which are capable of reducing the dynamic viscosity of the PSA while also imparting other beneficial properties into the PSA. For example, U.S. Pat. No. 5,162,410 discloses a hot-melt PSA comprised of (i) a silicone resin, (ii) a silicone fluid and (iii) at least one phenyl-containing polysiloxane fluid having a viscosity at 25.degree. C. of from about 5 to 60,000 centistoke. The compositions of U.S. Pat. No. 5,162,410 are disclosed as being more compatible with certain drugs and other organic materials than are the hot-melt PSA's of U.S. Pat. No. 4,865,920.
EP Application No. 0 443 759 discloses a hot-melt PSA comprised of (i) a silicone resin, (ii) a silicone fluid and (iii) a non-flammable hydrocarbon having weight average molecular weight of from about 300 to 1500. The compositions of EP Patent Application No. 0 443 759 are disclosed as being more compatible with certain drugs and other organic materials than are the hot-melt PSA's of U.S. Pat. No. 4,865,920.
These hot-melt compositions have been found to be inadequate for the delivery of hydrophilic drugs from transdermal drug delivery systems. There are several advantages to having a hot-melt composition that is hydrophilic. One advantage is that higher dosages of hydrophilic drugs can be employed without destroying the pressure sensitive adhesive. Another advantage is that the amount of drug released can be increased or controlled. Finally, the conditions under which a patch can be worn are greatly improved.
It is an object of this invention to provide a hot-melt silicone pressure sensitive adhesive composition that has improved hydrophilic characteristics while maintaining the pressure sensitive adhesive properties of shear, adhesion, and release.